1. Field of Invention
The invention relates to a thermal change computation apparatus for use with a machine tool, for computing amount of thermal change that is generated on the machine tool.
2. Description of Related Art
Machine tools include generally a machining mechanism for cutting or drilling a workpiece and assembling product component parts on a base plate, and a driving mechanism for changing a relative position between the machining mechanism and a workpiece. In general, machine tools for cutting a workpiece or the like include a holding mechanism for holding tools such as drills and taps, a main driving mechanism for rotating tools held on the holding mechanism, an X-axis feed mechanism for feeding tools in the X-axis direction, a Y-axis feed mechanism for feeding tools in the Y-axis direction, a Z-axis feed mechanism for feeding tools in the Z-axis direction, and an electronic control apparatus for controlling these feed mechanisms.
For example, as shown in FIG. 13, a machine tool is covered with a splash guard 100 for preventing cut wastes from splashing. In the inside of the splash guard 100, there are disposed a table 101 for holding a workpiece (not shown), an automatic tool change (ATC) magazine 102 for changing tools, such as drills and taps, and a machine tool body 103. The splash guard 100 provides a control panel 104, a work change opening 105 for loading or ejecting a workpiece and an inspection hatch 106 used mainly for maintenance.
As shown in FIG. 14, the body 103 includes a main shaft 107 for holding tools such as drills and taps, a main shaft motor 108 for rotating the main shaft 107, a ball thread mechanism 109 which comprises a nut 109a housing a number of steel balls and which is fixed to the main shaft side and a ball thread 109b inserted into and engaged with the nut 109a, a Z-axis motor 110 for rotating the ball thread 109b, a guide rail 111 disposed in parallel with the ball thread 109b, a slide 112 for coupling the guide rail 40 and the main shaft 28 (not shown).
In this body 103, the ball thread mechanism 109 and the Z-axis motor 110 constitute the Z-axis feed mechanism for feeding the main shaft 107 in the Z-axis direction. When the Z-axis motor 110 rotates the ball thread 109b, the main shaft 107 is moved in the Z-axis direction. Moreover, the table 101 may be moved in the X-axis direction and Y-axis direction. As the main shaft 107 may be moved in the Z-axis direction, relative positions among a workpiece and tools in the X-axis, Y-axis and Z-axis directions may be changed.
In this type of machine tool, as the ball thread mechanism 109 is operated, a frictional heat is generated and the ball thread 109b expands. Also in other mechanisms, a thermal change occurs due to the heat thus generated. When such thermal change occurs in the Z-axis direction, for example, an error occurs in a depth of flute and a height of stepped portion formed on a workpiece. If a dimensional tolerance of the workpiece is sufficiently larger than a thermal change amount, then a machining error caused by such thermal change is negligible. If not, such a thermal change should be corrected.
Therefore, there is provided a thermal change computation apparatus for computing a thermal change amount of a machine tool in Japanese laid-open Patent Publication No. 62-88548. In this computation apparatus, when a drive mechanism is controlled in accordance with a predetermined work program, a machining error is corrected in response to the computed thermal change amount.
However, in this machine tool thermal change computation apparatus, since the thermal change amount is continuously computed while the machine tool is operated, a system for executing such computation processing should be operated constantly as well, resulting in an increase in the computation processing load.
In view of the fact that a heat generated amount and a heat radiated amount are placed in the equilibrium state as the temperature rises while a machine tool is continuously operated, a method of computing a thermal change amount was proposed in Japanese laid-open Patent Publication No. 10-143217.
That is, during a period in which the machine tool is operated, a thermal change amount at each timing point is computed based on a saturated thermal change amount (thermal change amount in the equilibrium state) and a machine tool driving time. When the computed thermal change amount becomes nearly equal to the saturated thermal change amount, the value of the saturated thermal change amount will be used as the thermal change amount. In this case, if an accurate saturated thermal change amount is given, a thermal change amount at each timing point may be computed, thus reducing a computation processing load.
However, in the above thermal change computation apparatus, a saturated thermal change amount is fixed. When the machine tool operating program, which includes a step for driving a machine tool at a high speed and a step for driving a machine tool at a low speed, is executed in the apparatus, the thermal change amount computation accuracy is slightly lowered. Since the saturated thermal change amount is fixed relative to an average driving speed, it is unavoidable that the above computation method computes the thermal change amount as a smaller thermal change amount in the step of driving a machine tool at a high speed, and computes the thermal change amount as a larger thermal change amount in the step of driving a machine tool at a low speed.